The following publications are representative of the most relevant prior art known to the Applicant at the time of the filing of this application:
______________________________________ United States Patents ______________________________________ 2,559,343 Caton July 3, 1951 3,193,399 Washburn July 6, 1965 3,639,101 Washburn February 1, 1982 3,679,444 Washburn July 25, 1972 3,846,144 Parsons et al November 5, 1974 3,960,581 Cutler June 1, 1976 4,060,424 Hofmann November 29, 1977 4,093,470 Cherry June 6, 1978 4,331,771 Washburn May 25, 1982 ______________________________________
All of the above listed patents, Caton, Parsons et al, and Cherry, show various compositions useful as refractory cements or the like. Caton describes an alumina and silica refractory wherein a small amount of a mineralizer such as calcium oxide is used to encourage the growth of a bonding mullite. The Parsons et al patent describes the combination of an essentially graphite-alumina gunnable cement having an oxidation resistant bond wherein silicon is used to form one of the bonding components upon firing the composition in place. The silicon is fired together with the refractory mix which includes calcium aluminate and calcium silicate to bond the bauxite-graphite containing composition, wherein the bauxite may include an iron oxide component.
Cherry shows an alumina refractory bonded with a volatilized or fumed silica binder. In this patent the silica content is prepared in an acid slurry and is then mixed with the refractory component in order to promote a solution of at least a part of the silica in the water to enhance its bonding properties. After this step the mixture is then formed, rammed, or gunned, and fired.
The Washburn patents and the Cutler disclosure are of general relation to this invention for disclosing various aspects of the chemistry of the silicon nitride, silicon oxynitride reaction product used in the ramming cement herein disclosed. The Cutler teaching relates more particularly to a silicon nitride bond for an alumina containing refractory material.
The Hofmann patent shows a ramming cement within the field of this invention and, as such, its disclosure is incorporated herein by reference. This patent describes the preparation of a raw batch for such a cement having a large percentage of refractory grains including alumina, silicon carbide and the like, together with a low temperature setting bond of borosilicate glass, silicon oxide, and boron oxide. The low temperature bond may also include some alumina, silica, potassium oxide, calcium oxide, boric acid, zinc oxide, phosphorous pentoxide, zirconium oxide, titanium oxide and/or an inorganic fluorine compound. The low temperature bond sets within a range of between 350.degree. C. to 800.degree. C. to hold the refractory grains within a monolithic structure which retains its integrity up to and through the elevated range of temperatures at which the refractory grains sinter and other high temperature ceramic bonds form.
The present invention is an improvement on the Hofmann type of cement and provides a novel low temperature setting bond that includes a mixture of fumed silica, iron oxide, silicon powder, and calcium carbonate. When the low temperature bond is fired in a nitrogen or reducing atmosphere containing nitrogen, the silicon in the bond is converted to a silicon nitride and/or silicon oxynitride component that is distributed through the mass of the resulting cement to protect the mass from the attack of molten iron and slag flowing in troughs and runners lined with this composition.
This bond is particularly suitable to use for forming nitrides in situ during the initial sinter and as the cement is used. Iron oxide is a catalyst for the nitridation of silicon, and fumed silica and calcium oxide catalyze the reaction to form silicon oxynitride. In addition, these oxides combine with the boric acid to form a low temperature bond, and then catalyze the nitride reaction with silica.